Diagnosis for the wellhead pressure curve plays an important role in hydraulic fracturing technology. wellhead pressure can indicate the fracture propagation behavior during hydraulic fracturing. The fracture propagation behavior is closely related to the formation characteristics such as mechanical properties of reservoir rock, properties of the fracturing fluid, geo-stress conditions, and development of natural fractures. Therefore, by analyzing the wellhead pressure curve, hydraulic fracture propagation behavior can be recognized, and formation characteristics can be determined. Consequently, wellhead pressure diagnosis method can be applied to optimize hydraulic fracturing design as well as adjust on-site strategy.
Currently, scholars have mainly researched the characteristics of pressure curves of hydraulic fracturing in unconventional reservoirs. In 1981, Nolte and Smith categorized the logarithmic pressure curve into four typical classifications: I, pressure curve with a slope of about ¼, indicating that the fracture propagate along the length direction, which is a normal pressure curve; II, pressure curve with a slope of about 0, indicating that the natural micro-fractures in the formation might be opened so that the fluid leak-off volume equals to the injection volume, or the pressure exceeds the stress of the overlying formation (vertical stress), forming a T-shaped fracture; III, curve with a slope of about 1, indicating that the fracture propagation is restrained, and the pressure in the fracture rises sharply, and if the slope is higher than 1, indicating that a blockage (proppant screen out) has occurred in the fracture. In this case, appropriate action should be taken immediately to prevent proppant stuck in the wellbore; IV, curve with a negative slope, indicating that the fracture passes through the low geo-stress area, or the natural fractures are activated, increasing the fracturing fluid leakoff remarkably. Subsequently, many other scholars such as Yang Yue (2012) and Qu Guanzheng (2013) further subdivided the logarithmic pressure curve into six subcategories, and explained the pressure drop in the early stages of hydraulic fracturing. Scholars such as Martinez (1993), Fan (1995), Cao Xuejun (2002), Liu Xianling (2003), and Wang Zhengmao (2004) proposed a variety of analysis methods for hydraulic fracturing pressure curve by establishing inversion models for conventional reservoirs. Moreover, Wang Tengfei (2009), Zhang Ye (2014) and other scholars improved the fracturing pressure analysis method based on fractal geometry theory and neural network theory.
In recent years, the shale gas development scale has grown rapidly. However, because of the low porosity and ultra-low permeability of shale gas reservoir, conventional hydraulic fracturing in vertical well is unable to increase production enough to commercially exploit shale gas. Hence, the multi-stage and multi-cluster hydraulic fracturing in horizontal well, aiming at generating complex fractures network in tight but brittle shale formation, was invented and became the primary stimulation method in shale gas reservoir. Due to the strong heterogeneity of shale reservoirs, weaker mechanical structures (natural fractures and bedding fractures), and a large amount of fracturing fluid leakoff, hydraulic fractures usually propagate unevenly and discontinuously, forming complex fractures network. Therefore, the pressure curve of shale gas fracturing is dramatically different from conventional fracturing, it has more complex pattern with more multiple stages. Therefore, the pressure curve diagnosis method for conventional reservoir fracturing cannot be applied to shale gas reservoir fracturing. So far, unfortunately, there is not any specialized pressure curve diagnosis method for shale gas reservoir fracturing.
Thus, establishing a specialized pressure curve diagnosis method for shale gas reservoir fracturing is necessary, it can diagnose the extension behavior of underground fracture network and provide guidance for on-site operation adjustment during hydraulic fracturing to further enhance the recovery of shale gas reservoir.